***p <0.001.

2.3. Experimental materials and procedure

This study compiled a new set of materials to ensure that each image contains people and is purely emotional due to images from IAPS with or without people, as well as it specific emotional images affected neural responses of participants (Ferri et al., 2012; Weinberg & Hajcak, 2010). The formal experiment comprised 60 images (20 sad, 20 happy, and 20 neutral). These images of valence ratings indicated that happy images were rated as more pleasant (7.65±0.14, ps <0.001) than neutral (5.09±0.26) and sad images (2.46±0.26), and its also higher for neutral image compared to sad images (p <0.001). Additionally, the arousal score was higher for both happy (6.17±0.26) and sad images (6.17±0.26) compared to neutral images (3.05±0.22,ps <0.001), but no significant difference was found between the happy and sad images (p =0.98). All images had the same brightness and contrast, measuring 475 x 355 pixels, and were presented on a gray background.
The experimental procedure was adapted from the encoding-retrieval paradigm (Bone et al., 2020), which required participants to first memorize and then recall or imagine (see Figure 1). During encoding, each trial started with a 7-word Chinese title presented for 0.5 seconds, followed by the corresponding image displayed for 4 seconds (with the title remaining above the image). Between trials, a fixation cross was presented for 0.5 seconds. During imagery, each trial began with ”Ready” and remained visible until participants pressed the spacebar. This step allowed them to adjust their pace of the task, which is particularly beneficial for depressed participants (Zhou et al., 2021). Upon pressing the spacebar, a crosshair appeared for 0.5 seconds, followed by a title cue displayed for 0.3 seconds. After that, an empty frame with with dimensions matching the image (475 by 355 pixels) appeared for 3.5 seconds. Participants were instructed to vividly imagine the image corresponding to the provided title cue within the given frame. Subsequently, participants rated the subjective vividness and valence of their mental imagery on a 1–9 scale (max. 2 seconds). During the inter-trial intervals, a crosshair was displayed for 2 seconds.
All images randomly divided into 4 blocks, with each block containing 5 sad, happy, and neutral images. Each image was presented three times in a random order during encoding period and visualized three times in pseudo-random order according to title cue during imagery period; each picture was presented or visualized once before any image was repeated. To ensure participants were imagining the corresponding image, participants were required to indicate which of four exemplars they imagined in 7% of the catch trials (Dijkstra et al., 2018). Participants were asked to complete practice experiments before the formal experiment.